THE 15-20 μm EMISSION IN THE REFLECTION NEBULA NGC 2023

We present 15-20 mu m spectral maps toward the reflection nebula NGC 2023 obtained with the Infrared Spectrograph in short-wavelength, high-resolution mode on board the Spitzer Space Telescope. These spectra reveal emission from polycyclic aromatic hydrocarbons (PAHs), C-60, and H-2 superposed on a dust continuum. These emission components exhibit distinct spatial distributions: with increasing distance from the illuminating star, we observe the PAH emission followed by the dust continuum emission and the H-2 emission. The C-60 emission is located closest to the illuminating star in the south, while in the north it seems to be associated with the H/H-2 transition. Emission from PAHs and PAH-related species produces features at 15.8, 16.4, 17.4, and 17.8 mu m and the 15-18 mu m plateau. These different PAH features show distinct spatial distributions. The 15.8 mu m band and 15-18 mu m plateau correlate with the 11.2 mu m PAH band and with each other, and are attributed to large, neutral PAHs. Conversely, the 16.4 mu m feature correlates with the 12.7 mu m PAH band, suggesting that both arise from species that are favored by the same conditions that favor PAH cations. The PAH contribution to the 17.4 mu m band is displaced toward the illuminating star with respect to the 11.2 and 12.7 mu m emission and is assigned to doubly ionized PAHs and/or a subset of cationic PAHs. The spatial distribution of the 17.8 mu m band suggests that it arises from both neutral and cationic PAHs. In contrast to their intensities, the profiles of the PAH bands and the 15-18 mu m plateau do not vary spatially. Consequently, we conclude that the carrier of the 15-18 mu m plateau is distinct from that of the PAH bands.